Various approaches are provided for regenerating a NOx storage catalytic converter for an internal combustion engine. One example approach, as shown in U.S. Pat. No. 6,904,752 B2, comprises cylinder deactivation. The activated cylinder is operated here with a rich fuel-air mixture to achieve an enriched or rich exhaust gas composition for the purposes of the regeneration of the NOx storage catalytic converter. This ensures an increased exhaust gas temperature and also provides sufficient amounts of reaction partners for the oxides of nitrogen bound within the NOx storage catalytic converter.
In another example approach, an engine control system with which a multi-cylinder internal combustion engine can be operated is known from US 2003/0188527 A1. Therein, the inventors show that some of the cylinders can be deactivated, and in certain operating situations the internal combustion engine can be operated with a reduced number of active cylinders. During this time, for the purposes of the regeneration of a NOx storage catalytic converter, operating parameters of the active cylinders are adjusted such that the NOx storage catalytic converter is changed to a state that is suitable for regeneration.
In yet another example approach, an internal combustion engine and an operating method for the internal combustion engine are known from US 2014/0026872 A1, wherein some cylinders of the internal combustion engine can be deactivated as required and the engine continues to be operated during the deactivation with a remaining number of active cylinders. Control of inlet and exhaust valves of the deactivated cylinders is carried out such that the exhaust gas of the active cylinders can flow via a common exhaust manifold in the opposite direction from the outlet side of the deactivated cylinders to the inlet side of the deactivated cylinders. The exhaust gas flowing through the deactivated cylinders in the opposite direction can mix with the fresh gas for the active cylinders in the vicinity of a common inlet box. Exhaust gas recirculation thus actually takes place through the deactivated cylinders.
It is the object of the disclosure to describe a method with which NOx storage catalytic converters of lean burn engines can be efficiently regenerated under everyday operating conditions. For regeneration of a NOx storage catalytic converter of a lean burn engine it is necessary to pass a rich mixture with an excess of unburnt hydrocarbons and carbon monoxide to the NOx storage catalytic converter. This should in particular enable efficient regeneration at low loads. Furthermore, the torque output by the internal combustion engine during the transition phase from a lean burn mode to a rich burn mode for the purposes of the regeneration should be able to be operated in a controllable manner. Furthermore, high efficiency of the internal combustion engine should also be achievable during the regeneration phase and for preparation of the regeneration of the NOx storage catalytic converter. Furthermore, a time window necessary for the regeneration of the NOx storage catalytic converter should be able to be kept very short, which makes it necessary for the oxidation reaction for removing the oxides of nitrogen from the NOx storage catalytic converter to be able to proceed as quickly as possible.
The inventors herein have recognized the above issues and identified an approach by which the issues described above may be at least partly addressed. One example method for regenerating a NOx storage catalytic converter of a multi-cylinder internal combustion engine comprises deactivating at least one switchable cylinder of the internal combustion engine, maintaining at least one net valve and at least one exhaust valve of a deactivated cylinder partially open, routing a flow of fresh air through the deactivated cylinder, operating at least one active cylinder of the internal combustion engine; and routing a flow of exhaust gas from the active cylinder to the NOx storage catalytic converter, wherein during the regeneration of the NOx storage catalytic converter, a first part of an un-combusted gas emanating from the deactivated cylinder is added to the exhaust gas of the active cylinder, and a second part of the un-combusted gas is cooled via a cooling device and recirculated to the inlet side of the switchable cylinder and/or of the active cylinder.
A method for regenerating a NO storage catalytic converter of a multi-cylinder internal combustion engine comprising cylinder deactivation and comprising at least one cylinder that can be deactivated during the operation of the internal combustion engine comprises passing a flow of exhaust gas from the at least one non-deactivated (active) cylinder through the NOx storage catalytic converter, operating the at least one active cylinder of the internal combustion engine such that regeneration of the NOx storage catalytic converter can take place. During the period of cylinder deactivation of the at least one cylinder that can be deactivated, at least one inlet valve and at least one exhaust valve of the inactive cylinder are held at least partly open to enable a flow of fresh gas through the inactive cylinder; and a return flow of the un-combusted gas (fresh air) emanating from the inactive cylinders on the exhaust side back to the inlet side of the internal combustion engine. Admixing of at least a first part of the un-combusted gas recirculated to the inlet side of the at least one inactive cylinder and/or of the active cylinder(s) may take place. In addition, at least a second part of the un-combusted gas may be added to the exhaust gas of the at least one active cylinder.
A flow of exhaust gas through the NOx storage catalytic converter is reduced by the part of the un-combusted gas that is recirculated towards the catalytic converter. Owing to said reduction of the volumetric flow/mass flow through the NOx storage catalytic converter, the dwell period of the exhaust gas components of the remaining exhaust gas of the active/combusting cylinders in the NOx storage catalytic converter is extended, so that the redactions taking place in the NOx storage catalytic converter have more time available to proceed. Such a reduced space velocity improves the efficiency of regeneration of the NOx storage catalytic converter with rich exhaust gas.
Furthermore, there is a reduction in the amount of unburnt gas (un-combusted gas) in the exhaust system that exits the inactive cylinder(s). As a result, the concentration of the exhaust gas components that are necessary for the regeneration of the NOx storage catalytic converter is increased. With higher concentrations of said exhaust gas components, the chemical reactivity also increases, which overall causes an increase in the efficiency of regeneration of the NOx storage catalytic converter.
Due to a flow of the exhaust gas of the active cylinders through the NOx storage catalytic converter, the temperature of said exhaust gas in the NOx storage catalytic converter is higher, because said volumetric flow/mass flow is not diluted by un-combusted gases from inactive cylinders and thereby cooled. Because of said unwanted cooling by the un-combusted exhaust gases from the inactive cylinders, in the prior art with low loading on the internal combustion engine, the temperature of the exhaust gas flowing through the NOx storage catalytic converter is often too low to cause regeneration of the NOx storage catalytic converter. With the method according to the disclosure, a sufficiently high exhaust gas temperature is successfully achieved even for low engine loads, so even at low loads on the internal combustion engine, effective regeneration of the NOx storage catalytic converter is enabled.
Furthermore, with the method according to the disclosure, owing to the cylinder deactivation operation, remaining active cylinders may operate under greater load, which is usually effected with a richer mixture. Because individual cylinders are operated under higher load even at a lower total engine loading, the additional fuel cost to achieve a rich exhaust gas that is required for regeneration of the NOx storage catalytic converter is reduced. In the prior art, such an additional amount of fuel is usually added into the flow of exhaust gas downstream of the internal combustion engine by means of an additional injection device. This can be dispensed with when using the method according to the invention, or a significantly smaller amount of post-injection fuel can suffice to achieve effective regeneration of the NOx storage catalytic converter. This increases the overall efficiency of the internal combustion engine.
Furthermore, in the prior art it is often problematic to provide a well-controlled or accurately controllable exhaust gas composition for the regeneration of the NOx storage catalytic converter by means of post-injection. Since post-injection is no longer necessary for adjusting the exhaust gas composition with the method according to the disclosure, or has to be used to a significantly lesser extent, the exhaust gas composition is essentially predetermined by the exhaust gas emanating from the engine (from the active cylinders), it is simpler and more accurate to control and to adjust.
Furthermore, the composition of the rich exhaust gas that is necessary for the regeneration of the NOx storage catalytic converter during the regeneration phase can also be maintained better and in a simpler way with the method according to the disclosure.
A particular advantage of the method according to the disclosure is that, owing to the possibility of the metered addition of at least a part of the un-combusted gas that can be recirculated to the exhaust gas of the at least one active cylinder, the flexibility for adjusting and optimizing the exhaust gas temperature and the volumetric flow through the NOx storage catalytic converter during a regeneration phase of the NOx storage catalytic converter, and its optimization in said time period, are increased.
In an advantageous embodiment of the invention, at least the second part of the fresh gas emanating from the at least one inactive cylinder is controllably added from a recirculation line to the exhaust gas of the active cylinders upstream of the NOx storage catalytic converter in the direction of flow of the exhaust gas. With the said measure, the NOx storage catalytic converter is successfully provided on the input side with an optimized mixture of exhaust gas from the active cylinders and un-combusted gas exiting the inactive cylinders. Advantageously, such metered addition takes place at a sufficient distance upstream of the NOx storage catalytic converter in order to ensure adequate mixing of the exhaust gases from the active cylinders with the fresh gas emanating from the inactive cylinders.
In a particular embodiment of the method according to the disclosure, the un-combusted gas is controllably added to the exhaust gas of the active cylinders upstream of a turbine of a turbocharger. By this measure it is achieved that both exhaust gas components flow through the turbine of the turbocharger together, so that good mixing is achieved. On the other hand, the controllability of the exhaust gas properties of the exhaust gas composition that finally flows through the NOx storage catalytic converter is easier to influence or control if metered addition of the un-combusted gas already takes place in the “hot” exhaust system upstream of the turbocharger turbine.
In order to be able to provide a sufficiently rich exhaust gas composition for the regeneration of the NOx storage catalytic converter, it is recommended at least during the process according to the disclosure to operate the remaining active cylinders with a rich mixture, in particular with a mixture with air-fuel equivalence ratio (λ)<1.
According to a further advantageous embodiment of the invention, the un-combusted gas recirculated from the inactive cylinders, which is recirculated to the internal combustion engine on the inlet side, is cooled. This can in particular be necessary if for example there are inlet manifolds on the inlet side that cannot withstand the high gas temperatures. Furthermore, the fresh gas fed into the internal combustion engine is prevented from being heated in an unwanted manner.
In a further advantageous embodiment of the disclosure, more cylinders are deactivated with decreasing load or with decreasing revolution rate of the internal combustion engine. The result thereof is that in order to meet the current driver's wishes or the current loading of the internal combustion engine, the remaining active cylinders can be operated with a higher load and hence a richer mixture may be available in the exhaust gas (to be used for NOx storage catalytic converter regeneration).
In a further advantageous embodiment, fresh gas and exhaust gas or un-combusted gas emanating from inactive cylinders are conveyed to the cylinders that can be deactivated and the cylinders that cannot be deactivated separately from each other, at least in part and at least outside of the internal combustion engine. Said separation of the gas flows for example enables the un-combusted gas emanating from the inactive cylinders to be conveyed exclusively to the inactive cylinders on the engine inlet side or optionally, possibly by means of a suitable dosing valve, to also be mixed with the fresh gas for the active cylinders.
Advantageously, the metered addition of the un-combusted gas emanating from the at least one inactive cylinder to the exhaust gas of the active cylinder takes place on the outlet side by means of at least one valve device, for example a branching valve from the recirculation line into the exhaust system.
For example, the metered addition can take place continuously with a predetermined volumetric or mass flow. Nevertheless, the metered addition can also take place by means of a non-controllable valve by clocking the “open” and “closed” valve positions.
Appropriately, the metered addition can be controlled and/or regulated such that operating parameters that are necessary for the regeneration of the NOx storage catalytic converter are held constant or almost constant at least for a period of time that is necessary for the regeneration of the NOx storage catalytic converter. One such operating parameter is for example the oxygen concentration in the exhaust gas that is flowing through the NOx storage catalytic converter. Other operating parameters can be the exhaust gas temperature and the volumetric flow/mass flow through the NOx storage catalytic converter.
In order to carry out the above mentioned method for NOx storage catalytic converter regeneration, a multi-cylinder internal combustion engine is comprising at least one cylinder that can be deactivated may be provided, wherein the switchable cylinder that can be deactivated comprises an inlet manifold and an exhaust manifold. The inlet manifold and the exhaust manifold are fluidically connected by means of a recirculation line, wherein at least in the deactivated state (inactive state of the switchable cylinder), the recirculation line can carry a flow of un-combusted gas that is flowing through the switchable cylinder. According to the disclosure, the recirculation line is connected by means of a metering line to an exhaust system of the internal combustion engine in which the NOx storage catalytic converter to be regenerated is disposed.
Appropriately, a valve device is provided, by means of which un-combusted gas emanating from the inactive cylinder(s) can be added to the flow of exhaust gas from the active cylinders in a controlled manner. In this case the valve device can be a continuously adjustable regulating valve, with which an optional component between 0 and 100% of the un-combusted gas emanating from the at least one inactive cylinder can be controllably added to the flow of exhaust gas from the active cylinders upstream of the NOx storage catalytic converter.
In order to provide separation of the gas flows to be conveyed if needed, cylinders that can be deactivated and cylinders that cannot be deactivated each comprise common inlet manifolds and/or exhaust manifolds that are separate from each other.
In order to achieve a further increase in the flexibility of the exhaust gas composition that is provided to the NOx storage catalytic converter, it can be advantageous to associate each switchable cylinder from other switchable cylinders with a recirculation line from the exhaust manifold thereof to the inlet manifold thereof, wherein a valve device is disposed in at least one of the recirculation lines from the switchable cylinders, by means of which metered addition of the fresh gas emanating from the switchable cylinders to the exhaust gas of the active cylinders is possible.
In the case of a plurality of recirculation lines, it can be advantageous to admix different gas flows from the inactive cylinders to the exhaust system or the exhaust gas from the active cylinders by means of separately activated or separately regulated valve devices.
A switching valve may be used to feed the un-combusted gas from the inactive cylinders either into the exhaust system or into the intake system based on the valve position. If necessary, a cooler for cooling the recirculated fresh gas can be provided in the at least one recirculation line.
Furthermore, the device for performing the method appropriately comprises a control system, by means of which the metered addition of the un-combusted gas emanating from the at least one inactive cylinder to the exhaust gas of the active cylinders can be controlled and/or regulated by means of the valve device.